LIKE planet like moon. Saturn's moon Iapetus could once have had its own ring system, and it could explain the bizarre ridge that girdles this moon.

The ridge on Iapetus is a range of mountains unlike anything else found in the solar system: they are up to 20 kilometres high and run neatly around the equator.

One proposed explanation for the ridge is that Iapetus's spin suddenly slowed, changing the moon's shape and generating powerful stresses that forced up the ridge. But this seems unlikely, as the only obvious means of slowing the spin are tidal forces from Saturn, and Iapetus is too far out - about 3.5 million kilometres. It is difficult to explain how the tidal forces could have put the brakes on Iapetus.

Wing-Huen Ip of the National Central University in Taiwan now has an alternative idea: that the ridge could have come from outside the moon. The young Iapetus might have had rings of debris left over from its formation - a miniature version of the spectacular ring system around its parent planet. In this theory the isolation of Iapetus is an advantage, as Saturn's gravity would not easily disrupt the ring. Instead, the ring would gradually contract, ending up piled high on the moon's surface.

Astronomers will get a closer look at the ridge when NASA's Cassini spacecraft revisits Iapetus in September 2007, approaching to within 1000 kilometres.

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